In environments where there are many FMCW radars operating in the same vicinity and in the same frequency region, methods are necessary to reduce the interference such that the probability of an interfering signal being above threshold is low while simultaneously minimizing the total operating bandwidth covering all the radars.
One invention (Mehltretter U.S. Pat. No. 3,824,592) stochastically varies the frequency extent or introduces a time period of no frequency modulation. Neither of these techniques will work for a radar which needs the entire integration time during a required update rate, such as vehicle collision warning radars, for two reasons:
1. Non-modulated portions will reduce the integration time and thus the required detection range.
2. Changing frequency extent will change the range resolution.
Moreover with interfering radars at much less ranges than required detection ranges this single dimensional approach will not come close to providing interference levels below detection threshold.
This is especially true in applications where interference from any one scan will be sufficient to cause a significant loss in performance, requiring a very low probability of interference for satisfactory performance. Vehicle collision warning is one such application as a loss of information on any one scan of the environment will cause a significant loss of warning time.
Another invention (Chilak and Wehage U.S. Pat. No. 4,106,108) detects the approach of interfering signals towards an acceptable band pass of the radar with which it is associated. Their invention then shifts the phase of at least one of the radars in order to prevent interference. This requires coordination between the radars and only works with a small number of radars (their description covers just two radars). When there are many radars operating in the same vicinity, phase shifting is impractical. One such environment exists for vehicular collision warning radars in either an intracity or multilane highway environment. Here, the number of radars does not allow for coordination between the vehicles to reduce the interference.
In many of these applications, when FMCW modulation is used other factors (rain backscatter, required azimuth coverage, and multiple target separation) require a narrow beam scanning radar. Thus the problem resolves itself into two parts:
1. Minimizing the number of radars with interference levels above threshold, when in scan synchronization (FIG. 1). Subsequently, providing means to separate these interfering radars.
2. Minimizing interference when the scanning beams are not in synchronization with each other.
To solve this multiple part problem three techniques are used:
1. Spread spectrum FMCW signatures.
2. Power control as a function of angle.
3. Frequency band control either by external reference or by coordination between the radars.
Note that contrary to another invention (Brookner U.S. Pat. No. 3,787,853) here only one frequency is generated at any one time. Brookner's invention with multiple frequencies was designed to reduce clutter. Here the object is to significantly reduce interference from other multiple active radars.